1. Field of the Invention
Example embodiments generally relate to a light amount detector, a misalignment amount detector, and an image density detector, for example, for efficiently detecting an amount of misalignment based on detection of light from an image formed on an image carrier.
2. Description of the Related Art
Image forming apparatuses, such as copiers, facsimile machines, printers, and multifunction devices having at least one of copying, printing, scanning, and facsimile functions, typically form a toner image on a recording medium (e.g., a transfer sheet) based on image data using electrophotography.
In full color image formation, especially with tandem-type image forming apparatuses, it is important to minimize misalignment, between magenta, cyan, yellow, and black toner images formed on a transfer sheet. A tandem-type image forming apparatus includes four sets of optical writers and image carriers to independently form magenta, cyan, yellow, and black toner images, which often leads to misalignment between the individual toner images.
Generally, an optical sensor is provided to detect such misalignment. For example, the optical sensor uses a misalignment detection pattern formed on a transfer belt to detect an amount by which the color toner images are out of alignment with each other (misaligned). Based on that detection, the optical writer corrects image writing timing to prevent misalignment.
One example of related-art misalignment correction methods uses a diffused light sensor for detecting an amount of light. FIG. 1 is a schematic sectional view of such a related-art diffused light sensor 100R. The diffused light sensor 100R includes a light emitter 110R and a light receiver 120R, with respective optical axes 121R and 111R. A misalignment detection pattern 131R is formed on a transfer belt and conveyed in a sub-scanning direction D. When the light emitter 110R illuminates the pattern 131R, the light receiver 120R receives diffused light reflected from the pattern 131R.
FIG. 2 is a schematic view of several examples of misalignment detection patterns P1, P2, P3 . . . P9 (P1 to P9) provided on a transfer belt in a sub-scanning direction and a sensor spot SP of detection of the diffused light sensor 100R. In the detection patterns P1 to P9, black reference color patches K1 to K9 and yellow patches Y1 to Y9 are partially superimposed on yet offset from each other by different amounts. For example, in the detection pattern P1, the yellow patch Y1 is offset from the black patch K1 by a maximum amount of α, which equals a width of the yellow patch Y1, that is, the black patch K1 is not superimposed on the yellow patch Y1. However, in the detection pattern P5, the black patch K5 and the yellow patch Y5 are perfectly superimposed, so that the amount of misalignment α is 0, that is, there is no misalignment in position between the black patch K1 and the yellow patch Y1. In the detection pattern P9, the yellow patch Y9 is offset from the black patch K9 in a direction opposite to a direction of offset in the detection pattern P1 by the maximum amount of α. That is, in the detection patterns P1 and P9, the black patches K1 and K9 are offset from the yellow patches Y1 and Y9 in the opposite directions by the same amount α.
The sensor spot SP indicates an area of detection by the diffused light sensor 100R depicted in FIG. 1, and is substantially egg-shaped, as illustrated in FIG. 2, which results in a difference in sensitivity output of the diffused light sensor 100R detecting the patterns P1 and P9 having the same amount of misalignment α.
In addition, since both the optical axis 111R and the optical axis 121R are inclined toward a detection surface 130R in the direction D in which the transfer belt is conveyed, as illustrated in FIG. 1, the diffused light sensor 100R is blind to one side of the pattern 131R in a direction 141R and not blind to another side of the pattern 131R in a direction 142R. This results in a difference in sensitivity output of the diffused light sensor 100R detecting the patterns P1 and P9 having the same amount of misalignment α, causing a detection error in the amount of misalignment. As a result, the diffused light sensor 100R decreases the precision of detection.
Accordingly, there is a need for a technology to efficiently detect an amount of offset among different color patches.